Astronaut Gene Cernan became the last man to walk on the moon during NASA's Apollo 17 mission more than 40 years ago. After a life dedicated to pushing the limits of space travel, he died on Jan. 16, at the age of 82. (Jenny Starrs/The Washington Post)

When Neil Armstrong and Gene Cernan testified before Congress in 2011, it had been nearly four decades since man had set foot on the moon.

Armstrong was the first to do so. Cernan, in 1972, was the last.

So when the Obama administration canceled the Constellation program, which sought to once again place humankind on the moon, the astronauts who represented the bookends of the Space Race felt it was their duty to defend the boundless possibilities of celestial exploration.

Cernan said the program had been replaced by a “mission to nowhere,” reported AFP at the time, and that “we are on a path to decay.”

“We are seeing the book close on five decades of accomplishment as the leader in human space exploration,” Cernan testified before Congress.

“Neil and I aren’t going to see those next young Americans who walk on the moon. And God help us if they’re not Americans,” he said. “When I leave this planet, I want to know where we are headed as a nation. That’s my big goal.”

A year later, Armstrong died. On Monday, so did Cernan.

He was 82.

Not just during his numerous advocacy trips to Washington, but through his autobiography, a 2016 documentary film and in recorded interviews, Cernan spent his life challenging young people to eclipse his NASA legacy and put fresh footprints on the moon.

Cernan had famously predicted that, after the moon, man would be on Mars by the end of the 20th century. NASA says it still won’t meet that goal for more than a decade, a long delay in progress that became a point of consternation for the last person to set foot on the lunar surface.

U.S. astronaut Eugene Cernan, the commander of the final Apollo lunar landing mission in 1972, “the last man to walk on the moon,” died at the age of 82 on Jan. 16, 2017. (EPA/NASA)

For scientists and advocates of space exploration, Cernan’s death forced moments of reflection on the state of NASA and American ambition to discover what lies beyond Earth’s orbit.

Astrophysicist and cosmologist Neil DeGrasse Tyson weighed in as well.

“In 1927 Lindbergh flew from NY to Paris. 45 yrs later, in 1972 we last walked on the Moon,” Tyson wrote on Twitter. “45 yrs later, in 2017 we… we… we…”

The critiques come at a time of decreased funding for America’s space initiatives. In Cernan’s and Armstrong’s day, 5 percent of the federal budget was dedicated to NASA. Now, it receives less than 0.5 percent. Even so, the agency invested 30 years in the space shuttle program, helped build the International Space Station, launched efforts to further study asteroids and put a rover on Mars.

All that didn’t satisfy Cernan, though, who wanted more from NASA, and faster.

And of the dozen American men who walked on the moon during the Apollo era, from 1961 to 1972, only six are still living, reported the Associated Press.

The sense of profound loss that has thus far accompanied the passing of these astronauts is not just for the deaths of brave men turned American heroes, but for the Kennedy-era American exceptionalism that inspired the Greatest Generation to land on the moon in the first place.

It was a concept Cernan spoke of often, and one that motivated him to desperately shed his designation as “The Last Man on the Moon.”

He wrote an autobiography under that title in 1999, and closed the book with a passionate plea.

“Too many years have passed for me to still be the last man to have left his footprints on the Moon,” he wrote. “I believe with all my heart that somewhere out there is a young boy or girl with indomitable will and courage who will lift that dubious distinction from my shoulders and take us back where we belong. Let us give that dream a chance.”

Nearly 20 years later, in his final years, Cernan continued that fight.

“Even at the age of 82, Gene was passionate about sharing his desire to see the continued human exploration of space,” Cernan’s family wrote in a statement, “and encouraged our nation’s leaders and young people to not let him remain the last man to walk on the Moon.”

In a tribute to his old friend, Aldrin wrote that “Gene was probably the strongest spokesman for lunar travel and advocating a return to the moon.”

He was the last man there, Aldrin wrote, and “he wasn’t happy about that.”

“With the passing of the First Man – Neil Armstrong, and the passing of the Last Man – Gene Cernan, it is up to us Middle Men to carry on (the) spirit of Apollo into the future for our Nation and the world,” Aldrin wrote in the tribute.

How that “spirit of Apollo” translates to today’s ambitions for space travel, which lean heavily on the use of robots, remains a point of contention among American scientists and leaders. Some argue that technological developments have created a safer way to explore space until the time is right to send humans beyond Earth’s orbit, which will perhaps happen in the 2030s, when NASA hopes to put astronauts on Mars.

But others, like Cernan, were persistent in distinguishing the excitement stemming from putting robots vs. people into space.

“There is a difference between a space program that takes you to 300 miles away from your home planet and another one that sets you out on a voyage a quarter million miles away,” he said during an interview with NASA in 1991. “There are significant differences, both technologically and philosophically. And, quite frankly, I’m a little disappointed in us, at this time, to know that we’re really not much further along than we were back then.”

Nearly 20 years later, when he testified to Congress in 2010, Cernan implored the lawmakers to be “bold, innovative and wise in how we invest in the future of America.”

“Curiosity is the essence of human existence. Who are we? Where are we? What do we come from? Where are we going? Was there life on Mars? Is Mars like Earth is going to look in a billion years? Are we what Mars looked like a billion years ago. I don’t know. I don’t have any answers to those questions. I don’t know what’s over there and around the corner. But I want to find out.”

Even in his final words on the moon in 1972, after he’d planted an American flag there and carved the initials of his daughter in the dust, Cernan spoke with that same urgency.

As I take man’s last step from the surface, back home for some time to come – but we believe not too long into the future – I’d like to just (say) what I believe history will record. That America’s challenge of today has forged man’s destiny of tomorrow. And, as we leave the Moon at Taurus–Littrow, we leave as we came and, God willing, as we shall return, with peace and hope for all mankind. Godspeed the crew of Apollo 17.

"We leave as we came, and God willing, as we shall return, with peace and hope for all mankind."